1. Field of the Invention
The present invention relates generally to image forming devices, and more particularly, to a heat roller in a fuser of an electrophotographic image forming device.
2. Description of the Related Art
An image forming machine, such as a printer, copier, fax machine, all-in-one device or multifunctional device, typically includes a heating device, such as a fuser, to fix a developing agent, such as toner, to a media sheet. Most commonly, this is achieved with a two-roll fuser, wherein two rollers are coated with some sort of compliant material and both are coated with a release agent such as a Teflon® sleeve are brought into contact with some force, and the paper is fed between them. One roller is driven and one roller is idling, and one or both rollers is/are heated internally by means of a radiant heater, or sometimes inductive heating. Roll fusers are a well developed technology, frequently yielding good performance in terms of speed and quality of the fused image. The drawback is they have a high thermal mass, which translates to long warm-up times and higher energy consumption at startup. An alternate method of fusing is belt fusing. In a belt fusing system, a long, thin, flat heater, often made of ceramic, is supported by a rigid frame. A thin cylindrical metal tube, coated with a thin layer of compliant material and a release agent, encases the heater and frame assembly. The belt assembly is forcibly brought into contact with a driven compliant roll similar to that found in a two-roll fuser, and the paper is fed between the belt and roll. The belt slides over the heater surface by means of a lubricant. The minimal thermal circuit between the belt and the heater, and the low thermal mass of the belt cause these systems to reach operating temperature very rapidly, resulting in little energy wasted. However, the drawback in these belt fusing systems is that they inherently require high torque to drive them, due to the sliding friction between the belt and the heater.
Previous attempts have been made in the prior art to combine the best aspects of both types of fusing systems—the low thermal mass and minimal thermal circuit of a belt fuser, with the low friction and robust design of a two-roll fuser by the marriage of a resistance heater with a fusing roll. There are similar devices in industry, notably U.S. Pat. Nos. 6,940,045 to Sanpei et al., 7,026,578 to Mori et al., 7,024,146 to Kim et al., 7,046,951 to Kim et al., and 7,248,827 to Cho et. al. All consist of a metallic outer sleeve, a resistance heater, and a metallic inner core. All of these devices have a solid aluminum outer core, which carries with it increased thermal mass and correspondingly hampers warm-up time. All of these devices also focus on a common assembly process in that they are built from the outside in, by sliding the heater into the outer shell and locking the assembly together by fluid-forming the inner core resulting in the need for larger bearings which are more expensive than smaller bearings.
Thus, there is still a need for an innovation that will further reduce thermal mass and increase warm-up time and overcome problems encountered in the common assembly process.